Sudden Load Changes Research Articles

A super-twisting sliding mode observer for boost inverter-based hybrid photovoltaic -battery system control

This paper proposes an observer-based control scheme for a three-phase differential boost inverter in a hybrid PV-battery system. In a conventional control scheme for three-phase differential boost inverter (DBI)-based PV system, the measurements of input inductor current and voltage across output capacitors are required for obtaining the desired voltage at the load end. In a typical three-phase differential boost inverter operation, four voltage sensors and seven current sensors are required to achieve the desired AC voltage. In this paper, an observer-based strategy for elimination of these sensors without compromising power quality is reported. For estimation of boost inverter inductor current and output capacitor voltage, only DC-link capacitor voltage and the load currents are measured. A super-twisting sliding mode observer is used for estimation of the states of the boost inverter. A comparative study between the first-order sliding mode observer and super-twisting sliding mode observer is also presented. The simulation and experimental results show that the proposed observer-based control scheme works satisfactorily under various operating conditions such as sudden change of load, changes in solar insolation, and also under unbalanced load conditions.

GA Technique to Solve the AGC Problem of Thermal Generating Unit

In a large-scale power system, Automatic Generation Control (AGC) is responsible to area load changes and abnormal imprecise system operating parameters essentially means very fast minimization of area frequency deviations and mutual tie line power flow deviations of the areas for satisfactory and stable operation of the system. Some technique gives the good results in normal operation but in abnormal condition, it take large time to settle down the load disturbance, which is harmful for the system. New approach (Genetic Algorithm (GA) Technique) provides better control performance over frequency deviations and tie line power flow deviations due to a normal and abnormal operating condition of sudden load changes. In this paper six area model of thermal generating units has been developed and simulated in MATLAB Simulink software. Response of the developed model has been obtained by GA technique and compared with the other technique like; fuzzy, PID. Tabulated result shows that the GA technique give the better performance over the other technique due to settle down the frequency and tie line power flow deviations in less time and maintain the system constancy.

Synchronous Generator Control Concept and Modified Droop for Frequency and Voltage Stability of Microgrid Including Inverter-Based DGs

Microgrids have gained much attention in recent years. The main challenge of this system is controlling the voltage and frequency in islanded mode. The inverter-based distributed generators (DGs) have low inertial property and in load change, the microgrid frequency and voltage are easily violated. Using the synchronous generator (SG) control concept, the inertial property of SG results in more stability of frequency and voltage in microgrid. In other hand, droop method is the basic idea of control in islanded mode. However, the simple or modified droop method cannot guarantee returning the main parameters to the reference values when they are violated. Thus, combining the control concept of SG and also modified droop method can enhance the voltage and frequency stability and ensure returning the violated values to the reference ones. The main goal of this paper is proposing this combinational control while simulation results show that using the proposed control strategy, the stability of frequency and voltage is improved even in sudden and different load changes compared with other efficient works in this filed. Moreover, without control strategy, the instability in frequency and voltage may be undesirable in the proposed system as discussed in the paper.

Power swing detection and blocking of the third zone of distance relays by the combined use of empirical‐mode decomposition and Hilbert transform

This study aimed to develop a new method for detecting power swings and faults that may occur during these swings based on the combined use of empirical-mode decomposition and Hilbert transform. Power swings including line disconnection, fault occurrence, large sudden changes in the demand load, and reclosing, can cause a distance relay to malfunction and sever healthy lines from the network. Therefore, power swings and their simultaneous faults can pose a major challenge to protect the power systems. The proposed method in this study was tested on the standard IEEE 39-bus network for different power swing and fault states. It was also implemented on a distance relay manufactured by Vebko-Amirkabir Co. The evaluations show that the proposed method completely outperforms the conventional industrial method and other available methods in the literature. Since the proper functioning of this method depends on the choice of a threshold value, the current study also provides a simple and practical way to determine this threshold.

Virtual inertia support for wind turbine system

<span>Previously, a conventional synchronous generator is adapted into the wind energy conversion system to supply the required inertial support; however its slow behaviour may worsening the stability problem of the system during frequency event. In this paper, a new approach that enables virtual inertial support from supercapacitor during sudden load increase e.g. when wind turbine experiencing sudden wind or load changes is presented. Here, a new approach that can regulate the system frequency by controlling the charging and discharging process of supercapacitor is demonstrated. Hence, an algorithm on how to derive the behavior of supercapacitor and power converter is presented. From the simulation results, it has been found that this proposed approach successfully reduces the frequency nadir and ROCOF of the system frequency. It also able to avoid the second frequency dips during frequency recovery time.</span>

Improved virtual synchronous generator control for frequency regulation with a coordinated self-adaptive method

Power inverter adopting virtual synchronous generator (VSG) control can provide inertia support for distributed generation systems. However, it cannot take into account the dynamic regulation characteristics of frequency. Thus, when the system encounters a sudden change in load or disturbance, the dynamic process of frequency regulation will be greatly influenced. In view of this issue, an improved VSG control strategy based on a coordinated self-adaptive (CSA) method is proposed. The time domain analysis method is used to study the influences of virtual inertia and damping parameter perturbation on the system steady and dynamic performances. Further, in order to make the control strategy suitable for large load changes and suppress frequency variation beyond the limit, the secondary frequency modulation is introduced into the control loop. Through the coordinated adaptive control of virtual inertia, virtual damping and frequency modulation, the dynamic performance of VSG frequency regulation can be improved obviously. Simulation and experiment results have verified the effectiveness of the proposed CSA control strategy.

Disturbance Observer-Based Finite-Time Speed Control for Marine Diesel Engine With Input Constraints

In this article, a finite-time observer-based speed control strategy for the marine diesel engine is proposed with consideration of system parameter uncertainties, external disturbances and input constraints. Initially, a finite-time disturbance observer is designed, by which the synchronized uncertainties are accurately estimated in the sense of finite-time stability. Based on the observed information, a speed control strategy is proposed, and practical finite-time stability of the closed-loop system is demonstrated by rigorous theoretical analysis. Besides, for solving the input constraints inherent with the speed control system, an auxiliary variable is introduced to compensate the extra part of control output signals. Finally, numerical simulation and comparison are presented. The common operation situations of the marine diesel engines are taken into account, including starting, acceleration and deceleration between speed set points, sudden load changing, varied propulsion system parameters and different slope limits, etc. Comparing with the related existing method, the effectiveness and advantages of the proposed finite-time observer-based control law are illustrated.

Unified Control Strategy of Heterogeneous Thermostatically Controlled Loads with Market-based Mechanism

This paper studies the coordination of heterogeneous thermostatically controlled loads (TCLs) to provide the real-time ancillary services. A market-based control framework is adopted for its advantages. The first advantage is that the demand curve-oriented approach makes it possible to form a unified control scheme for heterogeneous loads without identifying their different characteristics. The second one is that the broadcast price signal helps simplify the downlink control and reduce the implementation cost. Then, the separate demand curve construction strategies based on a virtual price for different types of TCLs are presented. The flexibility of each TCL is reflected through the curve, and its practical constraints, i.e., comfort requirements of users and operation constraints of devices, are satisfied explicitly. To ensure the control fairness and full utilization for the regulation ability of TCL cluster, a comfort-level-equality principle is applied in demand curve construction. Simulations are carried out to verify the effectiveness of the proposed method in providing frequency regulation services, for which a regulation capacity estimation method is developed. Finally, a series of case studies are conducted considering the practical situations, e.g., model errors, imperfect communication and sudden load change after the end of services.

Tracking Power System State Evolution with Maximum-correntropy-based Extended Kalman Filter

This paper develops a novel approach to track power system state evolution based on the maximum correntropy criterion, due to its robustness against non-Gaussian errors. It includes the temporal aspects on the estimation process within a maximum-correntropy-based extended Kalman filter (MCEKF), which is able to deal with both nonlinear supervisory control and data acquisition (SCADA) and phasor measurement unit (PMU) measurement models. By representing the behavior of the state variables with a nonparametric model within the kernel density estimation, it is possible to include abrupt state transitions as part of the process noise with non-Gaussian characteristics. Also, a novel strategy to update the size of Parzen windows in the kernel estimation is proposed to suppress the effects of suspect samples. By properly adjusting the kernel bandwidth, the proposed MCEKF keeps its accuracy during sudden load changes and contingencies, or in the presence of bad data. Simulations with IEEE test systems and the Brazilian interconnected system are carried out. The results show that the method deals with non-Gaussian noises in both the process and measurement, and provides accurate estimates of the system state under normal and abnormal conditions.

Study on multi-function uninterrupted phase-separation passing system and its control method

At present, the sudden load change and the terminal voltage fluctuation are two mainly problems in the uninterrupted phase-separation passing (UPP) system. For further enhancing the reliability of the UPP system and compensate the voltage sag at the end of the traction power supply network caused by the suddenly connection of locomotive to the power grid, a multi-function uninterrupted phase-separation passing system (MUPPS) and its control strategy are proposed. It has the advantages of the linear load change and improving power supply reliability. Firstly, the topology and working principle of MUPPS are studied; secondly, the current variation characteristics of the bridge arm when the power is continuously supplied by MUPPS during the phaseseparation passing are analyzed. Then, the characteristics of MUPPS to provide reactive power and stabilize the terminal voltage of traction arm are analyzed. Then, the power transmission characteristics of MUPPS under emergency working condition are analyzed. Then, the control strategy in two conditions are proposed. Finally, simulation analysis and small capacity experiment are conducted. The theoretical analysis and test confirm the correctness and effectiveness of MUPPS.

Euclidean Metric based Fault Diagnosis in Power Transmission Line

This article proposes a unique algorithm for the saf ety of the power transmission line.In real time power transmission line has abnormal situations include such as sudden load changes, transient switching operations and so on etc., In the process of protecting power transmission line with associated power system equipment normal electromechanical relays are not reliable for faults protection as they fail to detect faults accurately. So, the euclidean distance technique approach can work effectively even at transient situations. The recommended methodology is progressively precise to recognize faults in the electrical transmission line. With help of MATLAB / SIMULINK software for a 300km,400kv electrical tra nsmission line, the accuracy of the proposed scheme was evaluate d by varying fault index and fault resistances.

Admittance Modeling and Stability Analysis for Inverter-Based Microgrid With Voltage-Source-Rectifier Load

Instability and low-frequency oscillation phenomenon of inverter-based microgrid (IBM) with voltage-source-rectifier load have obtained increasing concern. However, the influence of instantaneous angle of operation point on IBM is usually neglected during voltage faults, and it may generate the uncertain impact of voltage faults on the stable and safe operation of IBM. Besides, the influence of instantaneous angle of operation point may cause varying degrees of system oscillation and stability at a different angle of system states' change, including fault recovery. To solve this issue, a systematic approach of small-signal admittance modeling for IBM in the αβ frame is proposed in this article. The small-signal admittance model of each submodule (SM) of IBM is established and formed as a multi-input-multi-output (MIMO) system considering the actual closed-loop controls and decoupled strategies in the dq frame. Based on the proposed admittance models of SMs, the complete model of IBM is integrated and linearized, and the influence of both amplitude and instantaneous angle of operation point on the IBM stability is analyzed in small-signal model comprehensively. Besides, using the generalized Nyquist stability criterion (GNC), the influence of instantaneous angle in large-signal operation point on oscillation amplitude and stability of IBM is investigated. The proposed analysis approach can analyze and determine the responded oscillation and stability of the system more accurately during system states' variation, such as voltage sag and sudden changes in load. It also reduces the uncertainty of IBM behaviors during states' variation to ensure that IBM operates smoothly, flexibly, and stably. Finally, experimental results are presented to validate the correctness of the proposed analysis method.

Tracking technique for the sudden change of PV inverter load

Many power electronics applications require a power calculation in the control system. To get a suitable output, engineers need to control the process and regulate the power exchange with the grid. Since real and reactive power calculations are so crucial a topic, a novel control strategy for a single-phase photovoltaic (PV) inverter has been developed. Therefore, Direct power control (DPC) and a single-phase three-level space vector pulse width modulation (SVPWM) combine as a control and modulation system. In this paper, predictive real and reactive power control and SVPWM method are conferred in the inner loop. A voltage controller based on a proportionalintegral (PI) scheme is used in the outer loop to acquire constant output voltage and provide power refers to the DPC. The performance of the proposed method is verified by using MATLAB/SIMULINK.

An Induction Generator System Based on Instantaneous Torque Impulse Balance Control (ITIBC)

The induction machine, offering the merits of robust machine structure, brushless construction, and good voltage regulation capability, is a good choice used as a generator in the dc power system. For the induction generator, the instantaneous torque control (ITC) can achieve stable regulation of the output voltage and excellent dynamic performance of the electromagnetic torque. However, the proportional-integral controller of the voltage loop in the ITC can hardly guarantee the optimal dynamic performance of the output voltage, that is, during the response to the load sudden change, the output voltage exhibits several adjustments, overshoot, and relatively long recovery time. To solve this problem, an instantaneous torque impulse balance control (ITIBC) method is investigated in this paper. In the dynamic process, a group of optimal switch vectors is obtained to achieve the shortest recovery time and the minimum number of adjustments of the output voltage without overshoot. Thus, the optimal dynamic performance of the voltage can be achieved for the induction generator system. Simulation and experimental results verify the correctness and effectiveness of the ITIBC method.

Power Quality Performance Evaluation of Multilevel Inverter With Reduced Switching Devices and Minimum Standing Voltage

Multilevel inverters (MLIs) have been extensively employed to improve the power quality of the photovoltaic (PV) systems. However, the need for large number of components, higher standing voltage, and high harmonic content in the output of a conventional MLI greatly affects the system efficiency. Asymmetrical MLIs have been therefore developed as a suitable alternative to address these issues. This article aims at developing such a hybrid asymmetrical structure suitable for PV application that has a high level per component ratio and minimum standing voltage. The proposed MLI is assembled using a reduced switch H-bridge-based (RSHB) MLI structure with n asymmetrical repeating units and different level doubling circuit (LDC) combinations. The two dc sources used in the repeating units are in the ratio of 1: n voltage ratio, and using n such units, the proposed MLI structures, i.e., PS1 and PS2 can synthesize 4 n + 5 and 4 n + 7 levels, respectively, at the output instead of 2 n + 3 levels with only RSHB MLI. Comparative analysis reveals that both PS1 and PS2 have fewer switches, low standing voltage, less power loss, and lower cost. A 3.9-kW standalone solar PV system is considered for performance evaluation of the PS1 structure applying both the selective harmonic elimination and carrier-based pulsewidth modulation control schemes. In light of this, dc-link voltage balancing and self-voltage balancing mechanism of the LDC are warranted. Extensive simulation of the proposed MLI is performed in MATLAB/Simulink platform under a change in modulation index, sudden load change, frequency change, and step change in solar insolation. Furthermore, theoretical and simulation findings are validated experimentally by performing similar tests on a prototype of the proposed 17-level MLI.

Dynamic Performance Improvement of Doubly Salient Brushless DC Generator System With Controlled Rectifier

The doubly salient electromagnetic generator (DSEG) is gaining increasing attention in aircraft high voltage dc generator application due to its simplicity and de-excitation ability. However, the field winding inductance and the dc-link capacitor result in a larger electrical time constant. The dynamic response is decelerated, leading to difficulties in meeting aircraft electrical power standards. Minimum transient point tracking (MTPT) strategy based on the trajectory with minimum field current variation is proposed for the DSEG system with an angular position control (APC) controlled rectifier. Since the dc load current depends on both field current and the turn- off angle, the change in the field current is minimized and the dc load current is varied by controlling the turn- off angle. The trajectory of the optimized turn- off angle is studied to partly or fully fill the gap between the field currents at different loads. By tracking the MTPT trajectory, the change of the field current is decreased during the sudden load changes. The transient process is accelerated and the voltage variation is suppressed. In most operations, the voltage variation can be even neglected under both loaded and unloaded conditions. A 9-kW DSEG system based on controlled rectifier is implemented. The MTPT strategy is verified by experiments. The performances of the diode rectifier, controlled rectifier with fixed turn- off angle, and controlled rectifier with MTPT are compared. The dynamic performance is improved by the proposed method, making the DSEG more competitive in aircraft applications.

Utilization of electrochemical energy storage devices in autonomous power supply systems

Diesel and gas piston power units are used as a source of electrical power in autonomous power supply systems. For diesel power plants located in remote areas, the main component of the costs are the cost of imported fuel. For gas piston power plants operating on associated gas, the main problem is to ensure the stabilization of the frequency of alternating current with sudden load changes. The use of electrochemical electricity storage devices as part of power plants largely solves these problems. The article considers the choice of storage capacity for fuel economy, reduction the installed capacity of a power plant and frequency stabilization in the power supply system. It is shown that for diesel power plants, the use of electrochemical accumulators leads to both fuel savings and a decrease in its nominal capacity. For gas piston units, where the specific increase in fuel decreases with increasing load, the use of electrochemical accumulators reduces the installed power, but increases fuel consumption. The accumulator parameters for gas piston units are determined by the allowable overload/discharge of the load, and according to the energy intensity, they are determined by the duration of the maximum load, which is smoothed by the drive operation. We present numerical examples for estimating fuel consumption taking into account the efficiency in the charge-discharge processes of storage batteries. The simulation models show the impact of the electrochemical storage device on the stabilization of the frequency of an autonomous electric power supply system.

Optimizing Military Human-Robot Teaming: An Evaluation of Task Load and Modality Switch Cost

The effect of task switching on performance has been examined in many different fields and contexts. Sudden changes in task load can significantly impair performance, which can have detrimental consequences in dismounted military operations. As technology is advancing, robots are sought to take on the role of a teammate to the human soldier in the field. Robot-to-human communication modality may need to switch when mayhem occurs in military missions. Modality switching has been associated with performance decrements, although these effects are largely unknown in military human-robot teaming situations. The present study examined the cost associated with switching task demand and robot-to-human communication modality type on performance in a simulated cordon-and-search mission. The results showed that switches in task load affected threat detection performance. Auditory reporting increased performance more than visual reporting in low-after-high task load epochs. Performance with auditory reports was also higher in high-after-low demand blocks than low-after-high. The effect of switching needs to be taken into account for high-stakes human-robot interactions.

Rejection of effect of Non linearities in a PV Grid Integrated System using VSS SDLMS Scheme

The integration of photovoltaic system with existing utility plays a vital role to achieve the sustainable development goals set by renewable energy agencies. The grid integration takes place by following IEEE-519 standards. Number of control strategies have already been implemented to maintain the required standard. A number of issues are encountered during the process of synchronization, such as deterioration of quality of power flow due to sudden changes in loads, switching of power semiconductor devices and variation in solar insolation. To address these aforementioned issues a Variable Step Size Sign Data Least Mean Square (VSS-SDLMS) adaptive algorithm is proposed. The proposed controller makes the reference current adaptive to the adversities. In this paper the effective application of signal processing tool i.e.VSS-SDLMS for the power quality issues is well demonstrated by implementing in a three-phase single stage grid connected Photovoltaic (PV) system. The proposed scheme employs to mitigate the harmonic contents produced due to intentional and non-intentional non-linearities in the overall system to a desired level as per IEEE-519 and also it is maintaining the power factor to unity by controlling the reactive power flow. The transient performance of VSS-SDLMS is compared with the conventional LMS based algorithms. The overall system is modelled using MATLAB/SIMULINK.

Hierarchical voltage imbalance control for single‐/three‐phase hybrid multimicrogrid

With the rapid development of distributed generation (DG), microgrids and multimicrogrids (MMGs) appear at the end of distribution networks. For the islanded operation of a single-/three-phase hybrid MMG, a hierarchical coordinated control scheme is proposed in this study that includes primary and secondary levels. A compound control strategy based on quasi-proportional resonant and robust control is proposed for the primary voltage/frequency control to suppress voltage fluctuations caused by sudden changes in load power and DG output power; a coordinated strategy regarding tie-line power at the point of common coupling (PCC) is proposed for the secondary control to simultaneously improve voltage imbalance at the terminal of the master power supply, PCC, and load terminal. The proposed hierarchical coordinated control scheme is compared with conventional proportional–integral control and a control method based on the PCC to verify its performance. Test studies verify the superior robustness and voltage imbalance control performance of the proposed control scheme in coping with a sudden change in power and unbalanced loads.